The present invention relates to a method and apparatus for identifying the characteristics of a communication channel, and to a method and apparatus for restoring signals altered by the communication channel by using the channel characteristics information. In particular, the invention relates to a method and apparatus for substantially cancelling echoes (also referred to as "ghosts") which can occur during the transmission of television signals.
Communication engineering continually must deal with the problem of restoring a signal which has been altered by the communication channel over which the signal was transmitted. Signal restoration often can be achieved if the communication channel is fully characterized, at least as to those parameters which contribute to the signal alteration. Thus, a frequently essential component of the signal restoration problem is that of identifying the characteristics of the communication channel.
A straightforward approach to the channel identification problem is to transmit a known signal over the channel, and to receive the transmitted signal after it has passed through the channel. The originally transmitted signal is compared with the received signal, and a model of the channel characteristics is developed based on the comparison.
A good survey of the ghost cancellation problem and some theoretical solutions, can be found in W. Ciciora et al, "A Tutorial On Ghost Cancelling In Television Systems", IEEE Trans. on Consumer Elec., Vol. CE-25, No. 1, pp 9-44 (Feb. 1979) which is incorporated by reference herein.
The parent application describes a method and apparatus for restoring a received signal wherein no assumptions are made about the communication channel characteristics other than that the signal and channel interaction is described by linear system theory. Consequently, the channel is characterized completely by its impulse response. A test signal is transmitted over the channel and is received in a form distorted or otherwise altered by the channel, for example, by multipath interference. The test signal and the received test signal are related by the channel impulse response. Samples of the test signal are represented by an ensemble of sample sequences ordered in a matrix, called the test signal evolution matrix. The test signal evolution matrix changes from row to row in the manner that the test signal evolves in time. Samples of the received signal and the channel impulse response can be ordered in vectors. The channel impulse response sample vector can then be directly obtained through the multiplication of the inverse test signal evolution matrix and the received signal sample vector.
The sequence of channel impulse response function samples thus obtained by processing the test signal, serves to provide the correct sequence of coefficients to a filter which then is used to process the complete video signal in order to compensate for the channel echo and provide a substantially echo free video display.
When viewed in the time domain, the sequence of coefficients derived from a test signal reflects the concentrations of energy present in the received signal. The received signal will have one or more main echo components as well as numerous minor echo components. Most of the echo component energy is concentrated in the main echo components. The energy present in the minor echo components, while nominal for each individual minor echo component, can collectively cause a substantial amount of distortion in a television picture. Prior art echo cancellation devices utilize limiting methodology to extract from the received test signal, minor echo components which are below a certain level, for example those which are substantially close to zero. The limiting can alternatively be applied to the sequence of coefficients derived from the received test signal, rather than on the received test signal itself. The "limited" sequence of coefficients allows for the use of clustering algorithms which assign multipliers within the filter circuits, only to coefficients with non-zero values. As a result the energy present in those minor echo components which later translates into distortion in the signal, is never addressed during the processing of the complete received signal.
It is therefore an object of the instant invention to provide a method and apparatus for cancelling echoes in a manner which more effectively avoids the effects of minor echo components. It is another object of the invention to provide for effective grouping or clustering of coefficients without reducing the overall signal energy represented by those coefficients.